Surface treatment of steel



United States Patent 3,188,776 SURFACE TREATMENT OF STEEL Gilbert D. Dill, Mishawaka, Ind., assignor, by mesne assignments, to The Whcelabrator Corporation, Mishawaka, Ind., a corporation of Delaware No Drawing. Filed Aug. 20, 1962, Ser. No. 218,166 5 Claims. (Cl. 51-320) This invention relates to a process for the surface treatment of steel and, in particular, to the treatment of hot worked steel having mill scale thereon.

It is well-known that hot worked steel products acquire a substantial amount of mill scale which must be removed prior to most cold working operations. It has been the practice of the art to remove this mill scale by acid pickling, hot caustic salt bath treatment, abrasive blasting techniques or a combination of abrasive blasting followed by acid pickling. Some hot formed steels can be satisfactorily cleaned of mill scale by abrasive blasting only,

providing subsequent cold forming operations are not too Other steels such as stainless steel require acid' drastic. treatment even when blasted, to render its surface passive to corrosion as well as to clean the surface. In the case of hot formed plain carbon steels which are to be subjected to more drastic cold working operations such as cold rolling, abrasive blasting alone has not been satisfactory.

Specifically, when plain carbon steels cleaned by abrasive blasting alone are cold rolled, a heavy black smut has appeared on the surface, and this is an intolerable surface condition. This smut is believed to be caused by small particles of embedded mill scale, work hardened surface metal and amorphous iron on the surface breaking loose from said surface of the steel during the cold rolling operation and mixing with the rolling lubricantsas hereinafter more fully described. To overcome this situation, the prior art has resorted to a pickling operation following the blast cleaning. This pickling treatment, which is ordinarily applied for about fifteen seconds. effects the removal of these three surface defects whereby the appearance of smut during the cold rolling operation is essen tially eliminated.

The necessity for two cleaning operations of such a diverse nature is highly undesirable, and it is an object of this invention to provide a cleaning technique which eliminates the need for separate blasting and pickling operations.

It is a further object of this invention to provide an improved blasting technique for the surface cleaning of hot worked steel which eliminates the appearance of smut in cold rolled products.

It is an additional object of this invention to provide an improved blasting technique which eliminates the need for expensive acid pickling with its subsequent acid disposal problem which finds particular application in the surface treatment of hot worked steel sheet and strip which is to be subsequently cold rolled.

These and other objects of this invention will appear hereinafter, and it will be understood that the specific examples to be described are provided solely for purposes of illustration and not by way of limitation.

The process of the present invention is designed for the treatment of hot worked steel in preparation for a subsequent cold working operation such as cold rolling. The process generally comprises subjecting the hot worked steel to a first blast operation while utilizing'steel shot for the blasting material. This is followed by a second blasting operation wherein steel grit is employed as the blast ing material. By this double blasting technique, undesirable surface conditions are removed from the steel which, if not removed, would cause formation of smut on the surface during cold rolling.

It is believed that several conditions contribute to smut Patented June 15, 1965 formation, and the procedure of this invention serves to overcome these conditions. It will be understood, however, that applicant does not wish to be limited to any particular theory of operation. It is believed that the formaion of smut is caused by at least three contributing surface conditions. Thus, it has been observed that a flat hot rolled steel surface, when .blast cleaned with a steel abrasive, will be contaminated with microscopic particles of mill scale due to the impingement force of the abrasive particles. In the cold rolling operation. there is a tendency for these microscopic particles to be liberated and, upon mixing with the roll-ing lubricants, smut will be formed.

A second condition observed relates to the nature of the surface resulting from blast cleaning. Blast cleaning results in a surface composed of numerous hills and valleys, portions of which surface become work hardened duning the blast cleaning operation. These portions, particularly the hill or peak portions which are further work hardened during cold rolling, tend to disintegrate in the cold rolling operation and they mix with the rolling lubricants to form smut.

A third condition observed concerns the effects of the coiling operation at the end of the hot mill. It is believed that the scale on the steel surface during the coiling is affected by the temperature of the strip at this time. Coiling temperatures above about 1150 F. create a tougher scale as well as a deposit of molecular iron on the virgin metal surface beneath the scale layer. This deposit results from the reversal of the reaction which forms the FeO. This type of iron, which is sometimes referred to as amorphous iron, becomes chemically bonded to the virgin steel surface. It appears to work harden to a great degree, and a good deal of this iron can be removed by the single application of blast cleaning. However, the portion which remains will break down during cold reduction, mix with the rolling lubricants and form smut.

The first blasting operation is designed to remove substantially all of the mill scale by the mass-velocity impact shattering and abrasion action of the round steel shot. The second blasting operation serves to remove the more tenacious embedded scale, particles of work hardened metal and amorphous iron from the surface of the steel by acutting and securing cleaning action of the angular sharp edged steel grit. This combined blasting operation has been found to eliminate the need for pickling hot formed steel so that the subsequent cold rolling operation can be successfully carried out without the appearance of detrimental amounts of smut on the surface.

In the practice of this invention, strip cleaners of the type employing centrifugal blasting means are utilized when the concepts of this invention are applied to the treatment of sheet and strip. The Dill Patent Number 2,434,881 issued October 28, 1947, describes a strip cleaner of this type. A pair of strip cleaners, one utilizing shot and the other utilizing. grit, can be arranged in tandem and the cleaning operation carried out in a highly efficient continuous manner.

In a preferred form of this invention, the steel grit employed in the second blasting operation is heat treated to the extent that it will be harder than the shot employed in the first blasting operation. It is also preferred to employ shot having a diameter which exceeds the largest dimensions of the steel grit.

As used herein, the term steel grit" is to be distinguished from shot in that the shot generally exists in the form of pellets of rounded or curvilinear shape. On the other hand, grit is formed of steel particles of small dimension having irregular shapes and with edges that are relatively sharp by comparison with the generally rounded surfaces of shot.

velocity impact of the round shot and its shattering. and

abrasion action. A 'second cleaner is located tandem the first cleaner, and the centrifugal wheels therein direct grit onto the surfaces of the steel sheet or strip in order to remove the remnants of the mill scale, work hardened surface metal and amorphous iron. Greater'than*95 percent of the scale should be removed by the shotblasting operation, while the grit blasting attacks the remainder" of the smut forming materials.

As previously indicated, it is preferred that the largest dimensions of the grit particles be smaller than the largest dimensions-of the shot. The. steel shot may vary in size up to about 0.111 inch in diameter, while'the grit particles ordinarily have. largest dimensions'of less 0 than 0.1 inch. I'prefer to use. the smallest diameter steel shot that will insure shattering andbreaking up the mill scale upon impact. By so doing, greater surface coverage: is achieved per poundof abrasive thrown by the blasting.

units, less work hardening of the surface is done and the surface is not given as deep an etch which'faeilitates rolling out of the etch to a relatively smooth surface in subsequent operations. In a typical operation in' accordance with the practice of this'invention, the shot may have an average diameter between 0.017 and 0.039 inch, while the grit particles may have largest dimensions between 0.007 and 0.017 inch.

It is also preferred to have grit particles which are: harder than the shot employed; howeveix exceptionally hard grit particles are not necessary, and. this adds, to the economy of the process, since grit particles drawn back or tempered to a lower hardness have a. longer'lif'e- Shot which has been. found suitable for the practice of this invention ranges in hardnessbetween and Rockwell C, and preferably between 48 and 55 Rockwell C'. The grit employed has a hardness ranging between 55' and Rockwell C, and preferably between 58 and 64' Rockwell C. The use of grit having the lower of these values pro-- vides a particularly economical process.

As an example of a procedure involving the process of this invention, 18 inch wide 12 gauge low carbon steel is hot rolled and then introduced intov a. first strip cleaner.- employing centrifugal blasting wheels. The strip is then passed through the cleaner at a. rate between and' feet per minute, and the steel shot is directedv at thestrip' at a rate of about 1800 pounds per minute on each: side of the strip; The steel shot employedischaracteri-ze'dby" a hardness of 50 Rockwell. C. and an. average diam'eten between 0.017 and 0.039 inch.

The strip. is then immediately passed to a.- se'cond strip cleaner employing centrifugal: blastingywheels. Steel: grit with largest dimensions between 0.007 and 0.017 inch and having a hardness. of about 60 Rockwell C is employed. at. the same abrasive fiowrate. forth-is. secondsurfacetreatment. Upon cold rolling ofthe: steel the: appearance of smut is substantially eliminated;

In the. preferredform, both: the first and. the: second. blasting operationsare performed with steel abrasives inthe dry' state for reasons of simplicity: and economy. However, the se'cond-..bl'asti'ng operation; may. be. performed in a wet state by using a hard". grit suspended in liquids Hard, non-ferrous or non-metal c. grits such as. sand; or aluminum oxide may be substituted; for the steel grit...

In: viewof the results achieved by the practice of this. invention, it is believed apparent that the combi'nat n of the: shot and grit blast cl'ea-ni-ng'removes. the det-rri'mentwl surface conditions which are present when shot blast cleaning only is employed. The production of steel coming from a. cold rolling operation without. generation of. excess smut can, therefore, be accomplished by cleaning the. hot rolled steel using. mechanical. means only withoutrestorting to an acid pickle.

In considering the. concepts of this invention, it is apparent that various'alterations of the specific description provided can be conceived. Thus, the size and types of abrasive used, the. method of applying the blast and the cleaning. rates can vary while still. realizing the results. of this invention; It will also be. apparent that various other modifications can be made in the treatment described whichprovide the characteristics of this invention without departing from the. spirit thereof, particularly as defined in the following claims.

I claim:

1. A process for treating hot workcdsteel in'preparation for cold working comprising projecting hardened steelfshot, having smooth surfaces, at high velocity onto the surface. of the steel to disintegrate. scale and remove surfacev scale, and then projecting. hardened steel grit, havingsharp edges, onto the previously treated surface of the steelto remove substantially all of the remaining scale and other smut formingconditions remaining in the surface'of the steel, and in which the steel grit is of variable dimension with the average diameter of the steel shot exceedingthe. average of the large dimensions of the steel grit, and moving one of the elements includingthe surface of the hot worked steel and the source of the projection of thesteel shot and grit in rclativelinear movementfduring. projection of the. steel shot and grit onto the surface.

2. The process as claimed in claim 1 in which the steel grit is harder 'than'the steel shot.

3. A process in accordance with claim 1 wherein the diameters of said steel shot range between 0.017 and 0.03'9jinch and wherein the large dimensions of the particles of the steel grit range between 0.007 and 0.017 inch. 4. A process in accordance with claim 1 wherein the hardness of said steel shot ranges between 45 and 55 R'pckwell C and the hardness of said steel grit ranges between 55 and 65 Rockwell C.

5;. A process for treating hot worked steel in preparation for cold working comprising projecting hardened steel. shot, ha ving smooth surfaces, at high velocity onto the surface of the steel to disintegrate. scale and remove surface scale, projecting hardened steel grit, having sharp edges, onto the.- previously treated. surface of the. steel to remove substantially all of the remainingscale and other smut forming conditions remaining in the surface of the steel, in. which the steel shot has a diameter within the range of 0.017" to 0.039" inch. and the steel grit has its large dimension withi n'th'e range: of 0.007' to 0.01-7' inch and in" which'the: steel shot has. a hardness within the. range: of 45-55- R' ockwell C" and the steel grit hasv a hardness; within the'range: of. 5 5 65 Rockwell C'.

' References Cited" the-Examiner UNITE-D1 STATES PATENTS LESTER: ML. SWFNGJLE, Primary EXIlHliHPI.

; FRANK. BRONAUGH, Examiner; 

1. A PROCESS FOR TREATING HOT WORKED STEEL IN PREPARATION FOR COLD WORKING COMPRISING PROJECTING HARDENED STEEL SHOT, HAVING SMOOTH SURFACES, AT HIGH VELOCITY ONTO THE SURFACE OF THE STEEL TO DISINTEGRATE SCALE AND REMOVE SURFACE SCALE, AND THEN PROJECTING HARDENED STEEL GRIT, HAVING SHARP EDGES, ONTO THE PREVIOUSLY TREATED SURFACE OF THE STEEL TO REMOVE SUBSTANTIALLY ALL OF THE REMAINING SCALE AND OTHER SMUT FORMING CONDITIONS REMAINING IN THE SURFACE OF THE STEEL, AND IN WHICH THE STEEL GRIT IS OF VARIABLE DIMENSION WITH THE AVERAGE DIAMETER OF THE STEEL SHOT EXCEEDING THE AVERAGE OF THE LARGE DIMENSIONS OF THE STEEL GRIT, AND MOVING ONE OF THE ELEMENTS INCLUDING THE SURFACE OF THE HOT WORKED STEEL AND THE SOURCE OF THE PROJECTION OF THE STEEL SHOT AND GRIT IN RELATIVE LINEAR MOVEMENT DURING PROJECTION OF THE STEEL SHOT AND GRIT ONTO THE SURFACE. 